breaking spectrum gridlock with cognitive radios: an...

Breaking Spectrum Gridlock With Cognitive Radios: An Information Theoretic Perspective

Naroa Zurutuza - EE360 Winter 2014

Sunday, February 23, 14

Introduction

Cognitive Radio: Wireless communication system that intelligently utilizes any available side information about the activity, channel conditions, codebooks, messages or other nodes with which it shares spectrum.

Motivation: spectrum scarcity


Cognitive Radio Paradigms

UnderlayCognitive users transmit simultaneously with noncognitive users as long as interference is below threshold. Channel Side Information

OverlayCognitive users transmit simultaneously with noncongnitive users by assigning part of their transmit power to assist noncognitive users. Codebook Side Information

InterwaveIdentify available spectrum portions for a specific temporary location and time and use these spectrum holes for cognitive unlicensed transmissions. Activity Side Information

Primary UserOverlay CRUnderlay CR

Interference Threshold

Freq.

PSD

Interweave CR


Degrees of Freedom

• Not always possible to obtain capacity region of wireless network: asymptotic characterization using Degrees of Freedom

• Number of accessible interference-free dimensions

• At high SNR principal determinant of network capacity

CΣ(SNR) = d log (SNR) + o (log (SNR))

≈ d log (SNR)


http://en.wikipedia.org/wiki/Equals_sign#Approximately_equal


Interference Channel I

Goal: to determine capacity region of interference channels

Interference channel Rate-splitting


Underlay I

• Channel Side Information required

• Simultaneous cognitive and noncognitive communication below interference threshold --> Interference temperature

• Capacity: characterized by translating receive power constraint into transmit power constraint at cognitive transmitter

AWGN channelAverage interference constraint -->

corresponding transmit power constraint. Gaussian codebooks optimal

Gaussian MAC channel

Constraint in sum of transmit powersCapacity region: union of different MAC

with constant sum of powers


Underlay II

Techniques to fulfill the interference constraint:

• Multiple antennas: guide cognitive signals away from noncognitive users

• Spread spectrum / UWB

Degrees of freedom:

• Case of two MIMO users: (M1, N1) and (M2, N2) --> min(M1+M2, N1+N2, max(M1,N2), max(N1,M2))

• More than 2 users: at least 1/2 of total degrees of freedom (assuming global channel knowledge)


Overlay I

• Codebook Side Information required: problematic at the start of a communication

• Cognitive user applies encoding schemes to improve cognitive and noncognitive data rates

• Simplest overlay CR network:


Overlay II

Overlay encoding techniques:

• Rate-splitting: partial interference cancelation at decoders

• GP binning: precode against intereference

• Cooperation: relay noncognitive message to cognitive decoders

• Superposition coding: combination of the above techniques

Capacity-achieving regimes:

• Strong interference

• Gaussian channel with weak interefernce

• Common information


Overlay III

Degrees of Freedom:

• Two-user channel: 1 (interference channel with no cooperation) or 2 (2x2 MIMO with full cooperation in Tx and Rx)

• MIMO case (M1, N1) (M2, N2): min{M1 + M2, N1 + N2,(1 − 1T2)((1 − 1R2)max(M1,N2) + 1R2(M1 + N2)) + 1T2(M1 + M2),(1 −1T1 )((1−1R1 ) max(M2 , N1 )+1R1 (M2 +N1 ))+1T 1 (M1 +M2 )}

Cooperation:

• Relay channel

• Oblivious cognitive users

• Aware cognitive users, cooperative noncognitive users

• Full cooperation and cognition


Interwave I

Activity Side Information required: noncognitive user detection, sensing

• Signal degradations, fading, shadowing, dynamic nature of CR etc.

• Missed detection and False alarms --> ROC

• SNR Wall

Single Cognitive User link: two-switch model


Interwave II

Opportunistic Channel Selection:

• Narrow-band techniques: Frequency hopping, Frequency tracking

• Wide-band techniques: Frequency coding

Degrees of freedom:

• Perfect sensing: 1 --> CΣ(SNR) ≈ d log (SNR)

• Imperfect sensing: depend on duty cycle and false alarm/missed detection probabilities

Multiple User Case: Access protocols




Conclusions

• Cognitive Radio: tremendous promise

• Different paradigms, each require particular Side Information

• Difficult to provide exact capacity results --> degrees of freedom as metric of sum capacity of cognitive networks

• Cooperation to improve performance

• Research on combination of different paradigms


breaking spectrum gridlock with cognitive radios: an...

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